Television transmitting system



R. L.. CAMPBELL TELEVISION TRANSMITTING SYSTEM June 10, 1941 2 shards-sint 1 'Filed 'May 9, 1939 nnm 2.2.2.5

INVENTOR June 10, 1941. 4 R, 1 CAMPBELL 2,245,428.

` TELEVISION TRANSMITTING SYSTEM Filgd Mays, 1959 2 'sjheetks'heet -2 Patented June i0, 1941 TELEVISION TRANSMITTING SYSTEM Richard L. Campbell, Hasbrouck Heights, N. J., assignor to Allen B. Du Mont Laboratories, Inc., Passaic, N. J., a corporation of Delaware Application May 9, 1939, Serial No. 272,612

2 Claims.

My invention relates to improvements in television transmitting systems, and more particularly to those employing a cathode-ray tube for generating the video or picture signals,

In the operation of television transmitters of the character referred to, undesirable surges, tending to overload the video amplifier, are developed during the vertical return -line periods and the horizontal return-line periods. .Also, in the transmission of motion-picture film, the shutter action is accompanied by development of an inten-se surge which might; overload the video amplifier. A blanking circuit is therefore necessary in the early stages of the amplifier to prevent overload of succeeding stages by these surges. The blanking pulses must be of sufficient amplitude to neutralize the surge pulses from the transmitter cathode-ray tube. Heretofore, these surges have been blanked out by using a saturated tube in an early stage of the amplifier.v With this arrangement, however, the amplifier did not have the desired linear characterstic.

With the foregoing in mind, it is an object of my invention to provide an improved television transmitting system of the character referred to in which the so-called blanking can be effected without recourse to a saturated tube, and in which the video amplifier may be designed with more linear characteristics.

Other objects and advantages will hereinafter appear.

For the purpose of illustrating my invention, an embodiment thereof is shown in the drawings, wherein- Figure 1 is a simplified, block diagram of a television transmitting system constructed and operating in accordance with my invention; and

Fig. 2 is an enlarged, simplified, diagrammatic view showing details of the apparatus and circuits in Fig. 1.

In Fig. 1, the reference numeral I designates a cathode-ray tube of a conventional construction for generating the video signals, and associated .with which are the usual defiecting circuits and I2 for the horizontal and vertical sweeps, respectively. The video signals, amplified by a first amplifier I3, a second amplifier |4, and a line amplifier I5, are fed to a radio transmitter.

The deflecting circuits and I2 are controlled ates blanking pulses taken off by the output line I1 and which occur during the vertical returnline and the horizontal return-line periods. The

synchronizing signals are fed for transmission to the line amplifier I5 by a connection I8.

The amplifier I3 comprises the. tubes |9, 20, 2| and 22, and associated with these tubes on the same chassis are the tubes 23 and 24.

The video signals generated in the transmitte.` tube I0 are amplified in the two stages I9 and 2|) of video amplification and fed to the grid of tube 2|. The tube 24 is connected to the plate circuit of tube 2| by means of a coupling resistor 25. The signal at 26 is then impressed upon the tube 22 which has a cathode output circuit, as shown, connected to the video amplifierl I4 by connection 2l.

The signal from connection 2l is fed to the grid 28 of tube 24 by a connection 29, through a condenser 30.

The tube 24 has a common cathode resistor 3| with tube 23 which is operated under such conditions that the current drawn by tube 23 is sufficient to bias tube 24 beyond cut-off when no signal is impressed on the grid 32 of tube 23.

In operation, when the negative blanking pulses represented at 33 are impressed on the grid 32 to cut off tube 23, theexcessive bias is removed from the tube 24 to permit the same to draw current, so that for the duration of any blanking pulse, tube 24 functions as a negative feed-back device, which reduces the signal output by virtue of this degenerative connection or circuit. Thus, the signal, generated by injecting the blanking pulses into the video amplifier chain, does not overload the amplifier or cause saturation, such as would be the case if there were no provisions for degenerating this signal. as there are in my improved system.

The arrangement shown has feed-back action over only one stage, but greater feed-back action could be obtained by feeding back over three stages. In such case, instead of making connection to point 26 from the output circuit of tube 24, the connection would be made to the point 34.

It will be understood that modifications, within the conception of those skilled in the art, are possible without departing from the spirit of my invention or the scope of the claims.

I claim as my invention: l

1. Ina television transmitting system, a cathode-ray tube for generating video signals, an amplifier for said signals, a source of blanking pulses occurring during periods non-effective for vthe generation of the video signals, a degenerative circuit supplied from a stage of said amplifier and comprising an electron tube, a feedback connection from the output circuit of said electron tubs to the input cireuit o: s stage br said ampliner, and an electron tube supplied with said blanking pulses and connected to said nrst named electmn tube and being operable, in theode-ray tube for generating video simula. sn smpliiier for said siznsis, e source of `blanking pulses occurring during periods non-eii'ective tor the generation of the video simls. I. first electron tube supplied by and biased to cut-oi! by said blsnking pulses. and s degenerative feedback circuit from an output circuit to an' input circuit o! seid amplifier, said degenerative eircuit comprising s second electron tube controlled 10 by said ilrst electron tube.

- RICHARD L, CAMPBELL. 

